Project Summary/Abstract) The association of vitamin D deficiency, or low levels of circulating vitamin D, with increased risk for cancers, including breast cancer (BC), has received extensive attention. Experimental studies have shown that vitamin D has many anti-cancer properties, including anti-proliferative, anti-angiogenic, pro-apoptotic, and immunomodulation effects. Less attention has been paid to the role of vitamin D deficiency on BC prognosis and survival outcomes. Studies have shown that circulating vitamin D and polymorphisms in genes regulating vitamin D metabolism and signaling are both associated with BC risk, however, we know less about associations with BC prognosis. A few studies have linked low circulating vitamin D levels with prognostic factors for BC outcome, such as advanced disease or metastasis. Only one study directly evaluated the association between vitamin D status and BC survival, and no study has investigated this association in relation to vitamin D polymorphisms. We propose to address these questions in a prospective cohort of Chinese women with BC and test three hypotheses: 1) Circulating vitamin D level will predict BC progression and prognosis (recurrence, metastasis, overall or disease-specific death) after cancer treatment; 2) Genetic polymorphisms in vitamin D metabolism and signaling pathway genes affect circulating vitamin D level and its bioavailability in BC survivors; and 3) Circulating vitamin D and vitamin D gene polymorphisms together affect BC progression and prognosis. The proposed study will use resources from the Shanghai Breast Cancer Survival Study (SBCSS) and the Shanghai Breast Cancer Genome-Wide Association Study (GWAS). We will include 2,073 women newly-diagnosed with invasive BC and aged 20-74 years at the time of diagnosis, who have both blood samples for assessing circulating 25(OH)D and genotype information. Information on cancer diagnosis and conventional treatment, breast cancer recurrence, and causes of death will be verified through medical chart reviews. We will evaluate the effect of circulating vitamin D level and its combined effect with polymorphisms in the vitamin D pathway genes on BC progression and prognosis using appropriate statistical methods and controlling for known prognostic factors. The Mendelian Randomization (MR) method will be used to re-assess this association. Breast cancer survivorship is recognized as a critical component of cancer- related public health programs. The proposed study aims to fill a gap in our knowledge by evaluating the associations of circulating vitamin D and vitamin D pathway gene polymorphisms with breast cancer prognosis. This study will expand our understanding of the role of Vitamin D status and related pathway gene polymorphisms in BC prognosis. The study results could potentially lead to the development of new preventive and therapeutic strategies that can be applied to BC patients. In addition, the research could help determine if chemoprevention clinical trials with vitamin D should be considered for BC patients at high-risk for disease progression.